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Commit 9c23a131 authored by David Hildenbrand's avatar David Hildenbrand Committed by Christian Borntraeger
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KVM: s390: protect VCPU cpu timer with a seqcount 



For now, only the owning VCPU thread (that has loaded the VCPU) can get a
consistent cpu timer value when calculating the delta. However, other
threads might also be interested in a more recent, consistent value. Of
special interest will be the timer callback of a VCPU that executes without
having the VCPU loaded and could run in parallel with the VCPU thread.

The cpu timer has a nice property: it is only updated by the owning VCPU
thread. And speaking about accounting, a consistent value can only be
calculated by looking at cputm_start and the cpu timer itself in
one shot, otherwise the result might be wrong.

As we only have one writing thread at a time (owning VCPU thread), we can
use a seqcount instead of a seqlock and retry if the VCPU refreshed its
cpu timer. This avoids any heavy locking and only introduces a counter
update/check plus a handful of smp_wmb().

The owning VCPU thread should never have to retry on reads, and also for
other threads this might be a very rare scenario.

Please note that we have to use the raw_* variants for locking the seqcount
as lockdep will produce false warnings otherwise. The rq->lock held during
vcpu_load/put is also acquired from hardirq context. Lockdep cannot know
that we avoid potential deadlocks by disabling preemption and thereby
disable concurrent write locking attempts (via vcpu_put/load).

Reviewed-by: default avatarChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: default avatarDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: default avatarChristian Borntraeger <borntraeger@de.ibm.com>
parent db0758b2

arch/s390/include/asm/kvm_host.h

+8 −0
Original line number Diff line number Diff line
@@ -20,6 +20,7 @@ 
#include <linux/kvm_types.h>

#include <linux/kvm_host.h>

#include <linux/kvm.h>

#include <linux/seqlock.h>

#include <asm/debug.h>

#include <asm/cpu.h>

#include <asm/fpu/api.h>
@@ -553,6 +554,13 @@ struct kvm_vcpu_arch { 
	unsigned long pfault_select;

	unsigned long pfault_compare;

	bool cputm_enabled;

	/*

	 * The seqcount protects updates to cputm_start and sie_block.cputm,

	 * this way we can have non-blocking reads with consistent values.

	 * Only the owning VCPU thread (vcpu->cpu) is allowed to change these

	 * values and to start/stop/enable/disable cpu timer accounting.

	 */

	seqcount_t cputm_seqcount;

	__u64 cputm_start;

};

arch/s390/kvm/kvm-s390.c

+22 −8
Original line number Diff line number Diff line
@@ -1435,15 +1435,19 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu) 
static void __start_cpu_timer_accounting(struct kvm_vcpu *vcpu)

{

	WARN_ON_ONCE(vcpu->arch.cputm_start != 0);

	raw_write_seqcount_begin(&vcpu->arch.cputm_seqcount);

	vcpu->arch.cputm_start = get_tod_clock_fast();

	raw_write_seqcount_end(&vcpu->arch.cputm_seqcount);

}



/* needs disabled preemption to protect from TOD sync and vcpu_load/put */

static void __stop_cpu_timer_accounting(struct kvm_vcpu *vcpu)

{

	WARN_ON_ONCE(vcpu->arch.cputm_start == 0);

	raw_write_seqcount_begin(&vcpu->arch.cputm_seqcount);

	vcpu->arch.sie_block->cputm -= get_tod_clock_fast() - vcpu->arch.cputm_start;

	vcpu->arch.cputm_start = 0;

	raw_write_seqcount_end(&vcpu->arch.cputm_seqcount);

}



/* needs disabled preemption to protect from TOD sync and vcpu_load/put */
@@ -1480,28 +1484,37 @@ static void disable_cpu_timer_accounting(struct kvm_vcpu *vcpu) 
void kvm_s390_set_cpu_timer(struct kvm_vcpu *vcpu, __u64 cputm)

{

	preempt_disable(); /* protect from TOD sync and vcpu_load/put */

	raw_write_seqcount_begin(&vcpu->arch.cputm_seqcount);

	if (vcpu->arch.cputm_enabled)

		vcpu->arch.cputm_start = get_tod_clock_fast();

	vcpu->arch.sie_block->cputm = cputm;

	raw_write_seqcount_end(&vcpu->arch.cputm_seqcount);

	preempt_enable();

}



/* update and get the cpu timer - can also be called from other VCPU threads */

__u64 kvm_s390_get_cpu_timer(struct kvm_vcpu *vcpu)

{

	unsigned int seq;

	__u64 value;

	int me;



	if (unlikely(!vcpu->arch.cputm_enabled))

		return vcpu->arch.sie_block->cputm;



	me = get_cpu(); /* also protects from TOD sync and vcpu_load/put */

	preempt_disable(); /* protect from TOD sync and vcpu_load/put */

	do {

		seq = raw_read_seqcount(&vcpu->arch.cputm_seqcount);

		/*

		 * If the writer would ever execute a read in the critical

		 * section, e.g. in irq context, we have a deadlock.

		 */

		WARN_ON_ONCE((seq & 1) && smp_processor_id() == vcpu->cpu);

		value = vcpu->arch.sie_block->cputm;

	if (likely(me == vcpu->cpu)) {

		/* the VCPU itself will always read consistent values */

		/* if cputm_start is 0, accounting is being started/stopped */

		if (likely(vcpu->arch.cputm_start))

			value -= get_tod_clock_fast() - vcpu->arch.cputm_start;

	}

	put_cpu();

	} while (read_seqcount_retry(&vcpu->arch.cputm_seqcount, seq & ~1));

	preempt_enable();

	return value;

}
@@ -1704,6 +1717,7 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm, 
	vcpu->arch.local_int.float_int = &kvm->arch.float_int;

	vcpu->arch.local_int.wq = &vcpu->wq;

	vcpu->arch.local_int.cpuflags = &vcpu->arch.sie_block->cpuflags;

	seqcount_init(&vcpu->arch.cputm_seqcount);



	rc = kvm_vcpu_init(vcpu, kvm, id);

	if (rc)